1. Field of the Invention
The present invention relates to an IP telephone system, a network device, a communication method in disaster situations used therefore and an IP telephone terminal, and more particularly, to a communication method in disaster situations in a system.
2. Description of the Related Art
As shown in FIG. 12, an IP telephone system related to the invention includes IP telephone terminals 31 and 35, network gateways 32 and 34, and an SIP (Session Initiation Protocol) server 33.
In the IP telephone system, in case a disaster strikes at a site where the IP telephone terminal 35 resides, the IP telephone terminal 31 in a non-disaster area requests a call from the SIP server 33 in order to communicate with the IP telephone terminal 35 in the disaster area.
In the disaster situations, the SIP server 33 significantly restricts ordinary calls in order to avoid network congestion and reserve resources for important communications. In other words, the SIP server 33 restricts the number of calls from non-disaster areas to a disaster area.
Patent Document 1 (Japanese Patent Laid-Open No. 2006-304028) proposes congestion control on a network in disaster situations. A technique described in the Patent Document 1 encodes and decodes voice packets at a compression rate in accordance with a congestion state in the network in the event of congestion.
Patent Document 2 (Japanese Patent Laid-Open No. 2007-312299) proposes a technique for securing communication means of notifying a confirmation of safety between user terminals. According to the technique described in the Patent Document 2, when a disaster notification server device receives disaster area information indicating a disaster area on occurrence of a disaster, the disaster notification server device obtains a notification-registered user information list relevant to location information of user terminals in the disaster area, and connects a call between user terminals using an SIP server based on the notification-registered user information list.
However, there is a problem in the IP telephone systems related to the invention that it becomes difficult to make contact without contact means other than the telephone because calls flood to ask after people's safety in disaster situations, ordinary calls are significantly restricted and it becomes difficult to establish a call.
According to the technique in the Patent Document 1, it also becomes difficult to establish a call when ordinary calls are significantly restricted. According to the technique in the Patent Document 2, it is not dissimilar in that calls from the user terminals residing in the non-disaster area are significantly restricted, because the technique assumes calls between the user terminals in an area where a disaster struck.